Quad photoreceivers typically include a 2×2 array of p-i-n photodiodes with each photodiode followed by a transimpedance amplifier (TIA). Such quad photoreceivers are used in many applications, such as long baseline interferometry, free-space optical communication, missile guidance, and biomedical imaging and spectroscopy, which rely on free-space optical propagation with position and/or direction sensing capability. It is desirable to increase the active area of quad photoreceivers (and photodiodes) to enhance the link gain, and therefore sensitivity, of the system. However, the resulting increase in the photodiode capacitance reduces the bandwidth of the photoreceiver and adds to the equivalent input current noise, especially at high frequencies, for a given voltage noise level of the TIA.
In fact, the capacitance of the photodiode and its excess current noise arising therefrom, scales linearly as the device area, thereby negating the corresponding increase in the link gain. Owing to this contradiction, the front-end quad photoreceiver can limit the sensitivity of the overall system. An example of such an application is the Laser Interferometry Space Antenna (LISA), which proposes to detect gravity waves in space by measuring distance with ˜10 μm/√Hz accuracy over a baseline of five million kilometers. The present invention, as described herein relative to quad photoreceivers, is applicable for use in LISA which requires use of low-noise large-area photoreceivers, based on low-capacitance large-area photodiodes, for free space applications.
In LISA, the optical local oscillator (LO) power incident on each photoreceiver quadrant will be restricted to 100 μW to minimize the power requirements and thermal fluctuations for high pathlength stability. Assuming a photodiode responsivity of 0.7 A/W at 1064-nm wavelength, the desired shot noise-limited system operation requires the photoreceiver to display an equivalent input current noise density of <4.7 pA/√Hz per quadrant. For 0.9-A/W photodiode responsivity at 1550-nm wavelength, an equivalent input current noise density of <5.4 pA/√Hz is needed. Currently, LISA's sensitivity is restricted by the noise arising from 20- to 25-pF capacitance per quadrant demonstrated by typical 1-mm-diameter InGaAs quad photodiodes.
Accordingly, there is a need for photoreceivers having associated low capacitance photodiodes to achieve enhanced sensitivity. There is also a need for low capacitance photodiodes for many other applications that can be provided individually or in an array. The present inventors urge that such applications include systems relying on coherent optical detection, such as LISA, as well as those utilizing direct detection.